With the development of science and technology, the age of multimedia and computer have come. In a computer system, an input device, such as a mouse, keyboard, or trackball, plays an important role in controlling a computer device by a user. Most of these input devices work in a photo sensing way, as disclosed in U.S. Pat. No. 6,847,350 or U.S. Pat. No. 6,300,940, or work in a force sensing way, as disclosed in U.S. Pat. No. 7,855,715, U.S. Pat. No. 5,828,363 or U.S. Pat. No. 5,659,334.
However, as the rapid development of multimedia and various kinds of software, window interfaces of computers are getting more and more complicated and diversified, and therefore, the functions provided by the conventional input devices become failing to catch up with the demand of users. In order to enhance the functions that the input device can provide, a plurality of function keys are additionally built in the input device, or a function switch button is built in the input device for switching among multiple functions with a single function key. However, it is not convenient for a user to operate the aforesaid input device in practice. In view of this, the above issues of the conventional input device should be addressed.